Turbomachine compressors must increase the gas pressure therein. The compressors are divided into two main families, i.e., centrifugal compressors and axial compressors. Axial compressors are differentiated by their low compression ratio per stage and their very high volume flow, while centrifugal compressors have a higher compression rate. The compression is carried out in a certain number of stages, placed in sequence. Each stage may include of a rotary blade crown (15 rotors) and a stator blade crown (stator).
Two types of aerodynamic instability in the compressors may occur: surge and rotating stall. Surge is a violent aerodynamic phenomenon that intervenes in the compressors. Surge concerns aerodynamic instability that gives rise to considerable longitudinal waves and can extend up to a reversal in the direction of flow in the compressor. Surge is a phenomenon that may prove to be destructive for the compressor blades. Surge may be characterized by an out-and-out stalling of the compressor blades.
Rotating stall is also an aerodynamic instability affecting the compressor. Rotating stall is characterized by the presence of one or more localized fluid pockets (also called stall cells or pockets), spreading in the circumferential direction of the compressor, at a speed generally less than the rotation speed of the compressor. Thus, rotating stall corresponds to a partial stall of the compressor, which is characterized by a performance loss. The partial stall may be stable and may be translated by stagnation or unscrewing. Rotating stall generally appears during the start-up or re-ignition phases of the turbomachine and at the time of shutdown.
Rotating stall and surge are related, insofar as rotating stall can precede or co-exist with surge. At the time of start-up and shutdown of a compressor, the intermediate rotating speed, particularly lying between about 40% to about 70% of the rated rotation speed of the compressor, may cause a significant risk of rotating stall and surge. Discharge of a part of the air flow of the compressor via one or several anti-surge valves connecting the compressor to the exhaust may prevent this phenomenon.
Thus, it is known from U.S. Pat. No. 7,972,105, a compressor air flow extraction system and method over several stages with several valves that ensure a maximum extraction of the flow during the start-up or shutdown phases of the compressor, in addition to air regulation at the compressor inlet, in order to avoid the rotating stall areas. The valves are then closed once a rated speed is attained. FIG. 1 is a diagram representing the change in different cases of the pressure Ps ratio at the compressor outlet on the pressure Ps at the compressor inlet, according to the air mass flow at the compressor inlet, shows the effect of opening the valves that extract air from the compressor towards the exhaust. Air extraction may avoid the surge area, located above curve A, which is the operating curve limit of the compressor. Curve B1 is the operating curve when the valves are closed. Curve B2 is the operating curve when the valves are open.
In particular, air extraction avoids the critical area D associated with the critical point Pc (also called “pinch point” in English). Critical point Pc is the air flow value for which curve A is the closest of curve B1. It typically occurs for the rotation speed of the compressor lying between about 40% to about 70% of the rated rotation speed of the compressor.
However, the process described in U.S. Pat. No. 7,972,105 has as the disadvantage that it requires large sized air extraction pipes with high fluid speed and high fluid temperatures. These air extraction pipes lead to encumbering and costly devices and a risk of heavy vibrations therein. This invention aims at resolving this disadvantage.